Is Fast Charging for You?

Dec. 1, 2007
Yes, if you operate lift trucks at least 1.5 shifts per day, have ample charging opportunities, and can allow a full shift for batteries to be finish charged and equalized once per week.

By Nasser Kutkut, Ph.D.

Fast charging of industrial batteries is poised to become a mainstream charging technology due to the operational savings and the increased productivity and safety that this technology offers. Users are realizing the benefits of fast charging, as fast-charge systems are already buzzing at manufacturing plants and distribution centers around the U.S.

Fast charging is by no means a new technology. The concept of fast charging was first introduced in the 1970s with NiCad batteries in handheld applications. By the early 1990s fast charging was expanded to on-road electric vehicles (EVs) as a way to reduce charge time and make highway travel with such vehicles a reality.

Conventional vs. Opportunity vs. Fast Charging
Conventional Charging: With conventional charging, a battery is charged at a rate of 16-18A/100Ahrs, and it takes an average of 8-10 hours to fully charge the battery. Conventional chargers always charge the battery to a 100% state of charge (SOC) on a daily basis, a process that involves considerable gassing to bring the battery to name-plate specific gravity. This entails placing the chargers in centralized charging areas to maintain hydrogen concentrations below OSHA limits.

Conventional charging entails that the battery is charged over an 8-10 hour period, rests for another eight hours, and is used over an eight-hour shift. As such, conventional charging is ideal for one-shift applications where no battery changing is required. In multi-shift operations (two or three shifts) users need more than one battery per truck, which requires battery changing between shifts.

Opportunity Charging: Opportunity charging involves charging the battery at ~25A/100Ahrs charge rates whenever possible (e.g., lunch breaks and in between shifts). Due to frequent charging and to limit battery gas generation, opportunity chargers are normally set to charge batteries up to 80%-85% SOC throughout the day and back to 100% once a day (e.g., during night hours).

Opportunity charging is a good choice for extended shift operations where battery changing can be eliminated. In addition, opportunity charging extends the run time of aging batteries and recoups the lost capacity that comes with age (note the battery’s end of life criteria is when the battery capacity reaches 80% of its new value).

Fast Charging: In contrast to conventional charging, fast charging charges the battery at rates of 4060A/100Ahrs (three to four times conventional charge rates) at every opportunity possible. This includes breaks within and in between shifts. As such, a single battery per truck can be used, eliminating the need for additional batteries.

With fast charging, the batteries are only charged to 80%-85% SOC on a daily basis, thus eliminating any unnecessary gassing. Batteries must be finish charged and equalized (100% SOC) at least once a week, which is normally done on weekends.

Benefits of Fast Charging
In conventional charge applications, given a typical battery to truck ratio of 2-3:1, where a battery is used 8-10 hours over a 24-hour period, the battery utilization (asset utilization) is 33% on average, a poor utilization of assets. Due to the higher charging rate and more frequent charging throughout the day, fast charging requires only one battery per truck, improving truck driver productivity as it eliminates the time wasted in changing batteries in two- and three-shift applications. In addition, battery utilization factor jumps to a full 100% (full utilization of the asset).

The ROI and NPV of fast charging in two- and threeshift operations will exceed the return requirements of the most aggressive finance managers. Let’s consider the case of a manufacturing facility planning to acquire (or upgrade) a fleet of 50 vehicles. With conventional charging, assuming a ratio of 2.2 batteries per truck, 120 batteries would need to be purchased versus only 50 batteries with fast charging. In addition, with conventional charging battery extraction and changing additional equipment will be needed to change batteries between shifts.

Dr. Nasser H. Kutkut is co-founder and consulting CTO of PowerDesigners USA, LLC, Madison, Wis. He holds a Ph.D. in Electrical Engineering from the University of Wisconsin-Madison with a focus on fast charging.

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